The present invention relates to a hydrogen containing material and to a method for producing the material, and more particularly relates to a hydrogen containing absorbing material which is highly activated with hydrogen so as to be used as the negative pole material of the nickel-hydride cell, medium for storing and transporting hydrogen, catalyst for hydrogenizing carbon oxide and for converting it to hydrocarbon, medium for energy conversion, medium for recovering hydrogen gas from low concentration hydrogen gas and for purifying the hydrogen gas, and others, and to have the protective effect against the poison of the poisonous material (hereinafter called poisoning restraining effect).
The hydrogen containing (absorbing) material reversibly absorbs and discharges hydrogen by the treatment of heating, cooling, decompressing or pressuring thereof. Therefore, the hydrogen containing material is expected to become a storing material of hydrogen as a future secondary energy. Recently, the hydrogen containing alloy is used as the negative pole material of nickel-hydride cell and expected as a future high quality battery for the electric motor vehicle.
In order to stably cause the hydrogen containing material to absorb and discharge the hydrogen, it is necessary to carry out the initial hydrogenation treatment at high temperature, or high pressure, or high vacuum. For example, in the case of Mgxe2x80x94Ni alloy as the hydrogen containing material, the reaction vessel is evacuated at 350xc2x0 C., and the absorbing and discharging of hydrogen must be repeated over 10 times at 2-5 MPa. In the case of Laxe2x80x94Ni alloy or Laxe2x80x94Nixe2x80x94Al alloy as the hydrogen containing material, the reaction vessel is evacuated at 80-100xc2x0 C., and the absorbing and discharging of hydrogen is repeated over 10 times at 1-3 MPa. In order to keep the surface of the hydrogen containing alloy in very high active condition, the alloy must not be contacted with air. If the alloy is exposed to the air, the alloy is immediately oxidized so that the dissociation from hydrogen element to hydrogen atom is inhibited. Further, the hydrogen activity characteristic of the hydrogen containing alloy is remarkably reduced by a particle impurity gases included in the hydrogen gas such as CO, CO2, O2, H2o, NH3 and others.
Japanese Patent Publication 3-12121 discloses a microcapsule method of copper or nickel by the electroless plating in order to improve the thermal conductivity of the hydrogen containing material and to protect the material from impurity gases other than the hydrogen gas.
The Japanese Patent Application Laid Open Publication 5-213601 discloses treatment methods for highly activating and stabilizing the hydrogen containing material by treating the surface of the material using the supersaturation aqueous solution consisting of the fluoride metallic compound including alkali metal.
The Japanese Patent Application Laid Open Publication 8-9504 discloses material for hydrogen containing alloy which material is coated with electroconductive powder and cuprous oxide powder and with oxidation inhibitor by mixing the powder for hydrogen containing alloy, conductive powder, and cuprous oxide powder with a high energy mixer, in order to improve the initial hydrogenation characteristic and to maintain the characteristic for a long term, and discloses a method for producing the material.
However, none of the materials and method is proper for mass production on account of the installation cost, production efficiency and production cost. Although it is confirmed that the material has a protective effect against impurity gases other than the hydrogen of the hydrogen containing material, there are problems in stability and durability of the surface treatment layer at the absorbing and discharging of hydrogen.
At present, a hydrogen containing alloy is used for the negative pole material of the small secondary battery, and almost all alloys are AB5 alloys of the rare earth. As typical alloys, polyatomic alloys wherein the element A is La or rare earth metal alloys Mm (Misch-metall) and the element B is alloy produced by substituting Ni and a part of Ni with other elements (Co, Al, Mn, Si, Cr, Zr and others) are used. For example, there is alloys NaNi5, MmNi2.5, LaNi4.7Al0.3 and MmNi4.5Mn0.3Al0.2. The composing elements and composition ratio are selected in accordance with the using conditions. The hydrogen containing alloy is used not only for the secondary battery, but also for the chemical heat pump which uses the storage and the purification of hydrogen gas and the reaction heat of the alloy.
The reason why the rare earth AB5 alloy is substantially used is that the alloy can be initially activated with ease, has a great poisoning restraining effect, and can be easily treated compared with other alloys. However, the alloy is poor in durability. More specifically, the absorbing quantity of hydrogen reduces as the absorbing and discharging cycle increases. The alloy can not be used more than several thousand times. Therefore, although the alloy has a durability necessary for the negative pole material of the secondary battery, it is difficult to use the alloy for other fields which require much longer durability. Furthermore, there is a problem that the reduction rate of durability of the alloy further increases in the atmosphere at a temperature more than 150xc2x0 C.
As hydrogen containing alloys having at least one of the durability and a high hydrogen containing capacity, and having a possibility for highly balancing both the characteristics, there is titanium-base hydrogen containing alloy, zirconium-base hydrogen containing alloy, and vanadium-base hydrogen containing alloy, and others. However in spite of the fact that these alloys have the above described characteristic and do not deteriorate at high temperature, there are considerable number of alloys having difficulty in initial activation and sensitivity influenced by poisoning (atmosphere exposure, impurity gases in hydrogen gas such as CO, H2O, O2, H2S). As a result, these alloys can not exercise their inherent performance, and hence have problems in treatment thereof.
In order to improve reactivity, durability, hydrogen dissociation pressure-composition isothermal characteristic, and initial hydrogenation characteristic, polyatomic alloys are developed, which alloy is produced by substituting a part of a basic hydrogen containing alloy with another element. For example, a part of an alloy such as rare earth-base alloy, magnesium-base alloy, titanium-base alloy, zirconium-base alloy, or calcium-base alloy is substituted with another single element such as Al, Mn, Cr, Fe, or Cu, or with plural elements. However, an alloy having a remarkable protective effect against an impurity other than hydrogen is not developed.
In order to resolve the above described problems in the conventional alloys, the inventors of the present invention proposed highly activated hydrogen containing materials and the method for producing the materials, wherein on a surface of hydrogen containing alloy such as rare earth-base alloy, magnesium-base alloy, titanium-base alloy, zirconium-base alloy, or calcium-base alloy, a compound layer including fluorine is formed so as to highly activate the hydrogen containing alloy with hydrogen.
For example, in Japanese Patent Publication No. 2835327, there is disclosed a method for highly activating and for stabilizing the hydrogen containing material, wherein a hydrogen containing material and a hydrofluoric anhydride solution are contacted with each other so that a metallic fluoride film of the metal composition of the hydrogen containing material itself is formed on the material.
In the Japanese Patent Application Laid Open Publication 10-219301, there is disclosed a highly activated hydrogen containing material and a method for producing the material, wherein a hydrogen containing material including at least one of elements Al, Fe, Mg, Ca, Mn, Zn, Zr, and Li is fluorinated, thereby forming a fluoride of the metal on the surface or in a surface layer of the hydrogen containing material.
Furthermore in the Japanese Patent Application Laid Open Publication 10-219301, there is a disclosed highly activated hydrogen containing material and a method for producing the material, wherein a metal which becomes high active with hydrogen when fluorinated is preliminarily coated in the hydrogen containing material, thereafter the coating metal is fluorinated or treated so as to become fluoride. As a result, the hydrogen containing material becomes active with hydrogen.
In accordance with the method described in the Japanese Patent No. 2835327, it is possible to highly activate and stabilize the hydrogen containing material without a large installation and complicated steps. Therefore, the method has an advantage in mass-producing. However, there also exists a hydrogen containing material wherein a fluoride layer can not be formed on the material, or even if a fluoride layer can be formed on the material, the fluoride layer is impossible to become high active with hydrogen, because of the kind of the hydrogen containing material.
The former highly activated material and the producing method described in the Japanese Patent Publication 10-219301, has the same advantage as that of the Japanese Patent Publication No. 2835327. However, the metal which becomes high active is included in the hydrogen containing material itself. Therefore, only metals exposed on the surface of the hydrogen containing material are effective. If a small amount of a high active fluoride exists on the surface of the hydrogen containing material, the material has an effect though the amount is small. However, in the absorption and discharge reaction which accompanies a surface reaction, and in the methanization reaction which reacts H2 with CO, CO2 and others to them to hydrocarbon gas such as CH4, it is more preferable that a large amount of active portion exists on the surface. The hydrogen containing alloy is made into an alloy corresponding to the using condition of the alloy by adding another element to a basic composition element or substituting, with another element in accordance using temperature and pressure condition. Therefore, it is difficult to compose the hydrogen containing alloy only by metal which is highly activated by becoming fluoride. Consequently, such a hydrogen containing material can not be highly activated by the above described producing method.
In the latter hydrogen containing material and the producing method, since the hydrogen containing material is coated with a fluoride of high activity with hydrogen regardless of the composition element, the hydrogen containing material has a high reactivity with hydrogen. However, the hydrogen containing material as the matrix and the fluoride coating the material are basically different from each other in kind. Consequently, there may occur that the fluoride layer on the surface of the hydrogen containing material peels off because of expansion and contraction of the material at the absorption and discharge of hydrogen.
Accordingly, an object of the present invention is to resolve the above described problems in the prior arts, more particularly to provide a hydrogen containing material having a hydride layer on the surface, which hydride layer has a high reactivity with hydrogen, so that the hydrogen containing material is highly activated with hydrogen more than the inherent reactivity of the material despite a poisoning environment.
Another object of the present invention is to provide a hydrogen containing material having a fluoride layer which is not peeled off from the surface so that it is possible to maintain a high reactivity with hydrogen for a long term, while at least one of characteristics that is the durability of the hydrogen containing material itself and the high hydrogen absorbing capacity is maintained.
A further object of the present invention is to provide a method for easily forming a fluoride layer having a high activity with hydrogen on the surface of a hydrogen containing material so as not to peel.
According to the present invention, there is provided a hydrogen containing material the surface of which has layers comprising a first compound including the hydrogen containing material and fluorine, and a second compound including a metal which becomes high reactive with hydrogen when the metal becomes a compound including fluorine and a compound including fluorine, wherein the first compound and the second compound are integrally formed into a one-piece layer on the surface of the hydrogen containing material.
The hydrogen containing material comprises an ingot, or powder of a material or intermediate product or finished product of an alloy selected from a zirconium alloy, titanium alloy, vanadium alloy, rare earth alloy, and magnesium alloy.
Furthermore, the metal which becomes high reactive with hydrogen when the metal becomes a compound including fluorine is at least one metal selected from a rare earth metal, rare earth alloy, Fe, Al, Mg, Ca, Mn, Zn, Zr, Li, or alloys comprising these elements.
According to the present invention, the method for producing a hydrogen containing material comprises the steps of contacting a metal which becomes high reactive with hydrogen when the metal becomes a compound including fluorine with a fluorinating treatment liquid, thereby fluorinating the metal contacting a hydrogen containing material with the fluorinating treatment liquid contacted with said metal, thereby forming an integral layer comprising a first compound including the hydrogen containing material and fluorine, and a second compound including the fluorinated metal and fluorine on the surface of the hydrogen containing material.
The metal is melted in the fluorinating treatment liquid in a metal ion condition or in an ultrafine grain condition.
The fluorinating treatment liquid contacted with the hydrogen containing material is heated so as to vaporize the liquid to dry the hydrogen containing material.
The metal which becomes high reactive with hydrogen when the metal becomes a compound including fluorine is at least one metal selected from a rare earth metal, rare earth alloy, Fe, Al, Mg, Ca, Mn, Zn, Zr, Li, or alloys comprising these elements.
The fluorinating treatment liquid is a hydrofluoric acid aqueous solution or hydrofluoric anhydride solution, or solution composed by at least one of organic compounds such as piridine, triethlamine and isopopyl alcohol, and hydrofluoric anhydride.
It is possible to select a desired thickness of the metal fluoride layer on the surface of the hydrogen containing material so as to extend to a desired depth in accordance with the use of the hydrogen containing material.
In the hydrogen containing material of the present invention, in the cases that a large amount of metals which become high active when fluorinated are included in the basic composition elements, that a small amount of metals which become high active are included in the composition elements, or that highly activated metal is not included, in any case, it is possible to form a large amount of very highly activated fluoride layers, compared with a hydrogen containing material which is simply treated by fluorine, on the surface of the hydrogen containing material.
In the boundary surface between the hydrogen containing material as the matrix and the fluoride, a compound layer in which the concentration of elements composing the matrix and the concentration of the fluoride are changed in inclined conditions is formed. The surface of the material is in the condition the fluoride of the matrix and the fluoride of the highly activated metal are mingled.
In another case, in the boundary surface between the hydrogen containing material as the matrix and the fluoride, a compound layer in which the concentration of elements composing the matrix and the concentration of the fluoride are gradually changed in inclined conditions is formed, and the fluoride of the matrix is formed on the outer surface of the compound layer, and further on the outside of the outer surface, the fluoride of the highly activated metal is formed.
By forming the fluoride layer of metal the concentration of which changes in inclined condition, it is possible to prevent the metallic fluoride from exfoliating from the hydrogen containing material. Therefore, it is possible to maintain high activity with hydrogen for a long term in spite of a poisoning environment, while maintaining at least one of durability of the hydrogen containing material itself and a large capacity for absorbing hydrogen.
Furthermore, in accordance with the method for producing hydrogen containing material of the present invention, by contacting the treatment liquid for fluorinating the metal and the hydrogen containing material with each other, the metallic fluoride is formed on the surface or on the surface portion of the hydrogen containing material. Therefore, it is possible to produce the hydrogen containing material by a simple device with ease, and to correspond to the mass production.